In scenarios where we want to firsty check if a column mapping exists
and if we don't want do flow control with exception, it is very wasteful
to do
```
if (column_mapping_exists()) {
get_column_mapping();
}
```
especially in a hot path like `state_machine::apply()` becase this will
execute 2 internal queries.
This commit introduces `get_column_mapping_if_exists()` function,
which simply wrapps result of `get_column_mapping()` in optional and
doesn't throw an exception if the mapping doesn't exist.
d25be9e389